1. Field of the Invention
The present invention relates to process control and data acquisition systems. More specifically, the invention relates to integrated, fiber optically powered data acquisition, control and data transmission systems.
2. Description of Related Art
Computerized control systems and data acquisition systems have been around since the 1960s. Conventional control systems are used to monitor and control plant process equipment used in industries such as aerospace, chemical and food processing, manufacturing, oil refining, pharmaceutical, power generation, pulp and paper, telecommunications, transportation, waste water treatment, and others. Conventional control systems gather information from a population of local sensors located throughout the plant. The sensors monitor the operational state of plant systems by sensing parameters such as pressure, temperature, flow, voltage, current, radiation, breaker position, and valve position. The sensors transmit signals (e.g. electrical, pneumatic, etc.) that represent these parameters through a transmission path to a central computer. The central computer receives the information as input for algorithms that control plant operations. The central computer also stores historical information in memory, and outputs the information to display terminals to keep the operators apprised of plant conditions. Signals generated by the sensors are typically 4-20 mA or 0-10 V analog, or a stream of digital bits acquired at an appropriate sampling rate. The signals are transmitted along copper transmission lines, or may be transmitted wirelessly in some of the more modern systems.
Many conventional control systems are capable of issuing alarms to alert the operators of an out-of-tolerance condition such as low pressure in a pipe or loss of voltage at a power supply. The system may also perform analysis and control functions, such as determining whether a pressure leak is critical and if so, initiating corrective action by closing an isolation valve. These systems can be relatively simple, such as one that monitors environmental conditions of a small office building, or very complex, such as a system that monitors activity in a jet aircraft or on a nuclear powered ship.
Conventional control systems pose a number of challenges for designers in applications where system components must operate with high reliability. Examples of high-reliability systems are mission-critical systems in military craft, and safety-related systems in commercial nuclear power plants. In these applications, the electrical systems must be qualified to withstand a diverse array of design basis criteria in order to ensure system integrity under extreme operating conditions. These design bases include electromagnetic and radio-frequency interference (EMI/RFI), electromagnetic pulse, electrostatic discharge, nuclear pulse, precipitation static, indirect and direct lighting, ground loop currents, high temperature, radiation, shock and vibration, and seismic events, to name a few. In addition, communications lines must satisfy security criteria to prevent wiretapping and eavesdropping. Extensive grounding and shielding schemes are commonly used to qualify the cabling and electronics to meet these criteria and to withstand design basis events. These schemes add to the cost of the system, and in aircraft applications undesirably add more weight.
The power requirements of conventional control systems can also be troublesome to designers. Some sensors, such as resistance temperature detectors or RTDs, require electrical power from remote sources routed over transmission lines. The potential exposure of the power lines to damage or interference further complicates system integrity. Battery-operated sensors can be used to reduce cabling, but the tradeoff burdens operators with periodic battery replacement and battery charger maintenance.
Conventional control systems also commonly include control loops containing a series of signal conditioning circuits for each individual sensor. These circuits, or modules, include voltage-to-current (or current-to-voltage) converters, lowpass filters, analog-to-digital converters, signal amplifiers, and/or PID controllers. Special enclosures may be needed for containing these circuits and shielding them from noise. In aircraft, the control circuits are inherently noisy, especially those operating at 400 Hz. Noise reduction circuits employing filters, differential amplifiers, ground loop isolators, optical isolators, and elaborate grounding schemes are needed to qualify the system. Despite judicious use of these features, the systems remain susceptible to noise-induced interference.
In more complex control systems, such as those used in large industrial plants or military craft, configuration control is another major concern. Each of the sensors and modules in a control loop must be periodically maintained and accurately calibrated to ensure plant operability. Laboratories are needed for testing and calibration. Technicians must be trained to perform the tests. Maintenance records must be kept current, often by manual entry, to ensure that the configuration of the plant is known at all times. That is, at any given time, every component in the plant must be readily identifiable by its model number and serial number, and the calibration and maintenance histories must be up to date and available for review. This places another substantial burden on plant operations.